1. Field of the Invention
The invention relates to a control system including an electric motor and a speed reducer that amplifies motor torque generated by the electric motor. An example of such a control system is an electric power steering system (EPS).
2. Description of the Related Art
A column assist-type electric power steering system in which an electric motor and a speed reducer are arranged at a column part has been known as an electric power steering system. Hereinafter, such a system will be referred to as a column-type EPS. The column-type EPS includes, for example, a steering wheel, a steering shaft, an intermediate shaft, a steered mechanism, an electric motor, and a speed reducer. The steering shaft is formed of an input shaft connected to the steering wheel, an output shaft connected to the intermediate shaft, and a torsion bar that connects the input shaft and the output shaft to each other. The electric motor is connected to the output shaft via the speed reducer.
A column in this specification means a part including a steering wheel, an input shaft, a torsion bar, and an output shaft. A power column in this specification means a part including a steering wheel, an input shaft, a torsion bar, an output shaft, a speed reducer, an electric motor, and a control device for the electric motor. A typical column-type EPS in this specification means a column-type EPS that does not have a function of compensating for friction of the speed reducer.
In a typical column-type EPS, motor torque generated by an electric motor is transmitted to an output shaft via a speed reducer. Hereinafter, the motor torque that is transmitted to the output shaft will be referred to as an assist torque. The assist torque transmitted to the output shaft is then transmitted via an intermediate shaft to a steered mechanism including, for example, a rack-and-pinion mechanism. The speed reducer is, for example, a worm gear formed of a worm and a worm wheel. Because friction generated in the speed reducer is large, a response of steering to a steering input may deteriorate due to the influence of the friction.
Therefore, a method of compensating for friction generated in a speed reducer has been developed in order to improve a response of steering to a steering input. A simplest friction compensation method is a method in which a certain value of frictional force is added to an assist torque command value depending on a sign of a steering velocity.
Japanese Patent Application Publication No. 2003-170856 (JP 2003-170856 A), Japanese Patent Application Publication No. 2000-103349 (JP 2000-103349 A), and Japanese Patent Application Publication No. 2003-291834 (JP 2003-291834 A) each describe a method of compensating for friction on the basis of an assist torque command value, which is computed on the basis of a detected steering torque detected by a torque sensor, and the detected steering torque. Specifically, the magnitude of a frictional force of a speed reducer is estimated on the basis of the assist torque command value. The sign of the frictional force of the speed reducer is determined on the basis of the detected steering torque. The frictional force of the speed reducer is estimated in this way. The value of the estimated frictional force of the speed reducer is added to the assist torque command value.